This invention relates to thermoplastic pipe and more particularly to an improved thermoplastic pipe stub end.
In assembling thermoplastic pipe into piping systems, successive lengths of pipe are generally either connected directly by heat welding or, where ready separation may be required, connected mechanically e.g. by bolting together metal flanges which seat against thermoplastic stub ends already heat welded to the ends of the lengths of thermoplastic pipe to be joined. A preferred method of heat welding is by butt fusion.
As used herein the term "butt fusion" means a method of heat welding comprising the steps of (1) aligning the thermoplastic parts to be joined; (2) bringing the mating surfaces of the thermoplatic parts to be joined into engagement with the opposite sides of a flate heater plate which is maintained at a predetermined termperature to plastify the surfces; (3) separating the plastified surfaces from the heater plate; (4) removing the heater plate; and (5) bringing the plastified surfaces together under a predetermined pressure.
In practice, where butt fusion is performed correctly, the direct or pipe-to-pipe heat welds are normally as strong as the pipe itself i.e. in tests as hereinafter described failure rarely occurs at the heat weld. However, where the same butt fusion technique is used to join standard thermoplastic pipe stub ends to thermoplastic pipe, the heat welds are frequently the point of iniation of failure when piping systems including such stub ends and pipe are tested to destruction in tests as hereinafter described.
The standard thermoplastic pipe stub end, as further described and illustrated hereinafter, consists of a flange and a neck portion, which is adapted to be heat welded to a thermoplastic pipe. The length of the neck portion is approximately equal to the width of the flange and the outer circumference of the neck portion joins the flange in a smooth arc.
In order to facilitate the formation of stub end-to-pipe welds which are more nearly equivalent to pipe-to-pipe welds, the trend in the art has been towards the use of thermoplastic pipe stub ends having longer neck portions. Thermoplastic pipe stub ends having longer neck portions with a continuous taper from the flange to the welding face (i.e. the neck wall is very thick at the flange but reduces to near pipe diameter at the welding face) have also been used. However, thermoplastic pipe stub ends having such longer neck portions are expensive to produce and to maintain an inventory thereof.